Anterior cruciate ligament (ACL) injuries occur at a 2 to 10-fold greater rate in females compared to males participating in the same sports. While many advances have been made in terms of surgical and rehabilitation treatments for ACL injured patients, long term outcome studies show that these patients are at a high risk for developing knee osteoarthritis 10-15 years after ACL injury regardless of the treatment. In order to design successful ACL injury prevention programs, underlying mechanisms linked to the higher risk of ACL injury in females compared to males need to be better understood. Our general hypothesis tested in this proposal is that musculoskeletal changes, that accompany rapid adolescent growth, are associated with the development of sex differences in neuromuscular control of the knee and lead to an increased risk of ACL injury. Detailed longitudinal analyses of females and males throughout rapid growth will be critical to identifying the mechanisms which lead to increased risk of ACL injury. This project will involve a secondary analysis of previously collected data on approximately 2000 young females and males between 12 and 18 years of age which participate in soccer and basketball. Specifically, we aim to determine the onset of growth-related sex differences that increase knee load and risk of ACL injury in adolescents. In addition, we will determine the anatomical mechanisms related to rapid growth that underlies the sex divergence in knee abduction moments in female athletes. Our project will have a significant overall impact on female health through the development and timing of intervention programs aimed towards young at-risk athletes. PUBLIC HEALTH RELEVANCE: An anterior cruciate ligament (ACL) injury can be devastating and places an athlete at a high risk of developing osteoarthritis long term. Unfortunately, surgical intervention does not change the odds of developing knee osteoarthritis after injury. We utilize an innovative approach with established biomechanical methods to identify mechanisms of ACL injury in order to develop effective interventions to prevent ACL injury.